Oceanographic measurements made with moorings are often plagued by the action of ocean currents upon the cables or ropes used to secure instruments at a depth. These cables, ropes or harness assemblies drift and/or strum in response to water flowing by them. Hydrophone arrays used to measure sound propagation in the ocean are extremely sensitive to the effects of strumming and angular deflections caused by the action of currents. This is largely because the measurement of the sound propagation vectors is obtained from the serial excitation of hydrophones along a linear or predefined spatial vector. Deviations from the assumed orientation and location of hydrophones in the array introduce errors directly into the estimation of the propagation vectors and can even inhibit the excitation of certain hydrophones. Strumming of a mooring or array can also introduce acoustic noise directly into the signal.
Depth sensors can be used to correct some of the errors but only a time varied three dimensional map of an array's shape will be able to determine all the anticipated shape variations. This can, in principal, be derived by the use of an array of tilt sensors attached to the mooring and along the array but practice has shown that this approach is plagued by a number of hidden problems. These types of sensors are generally large and expensive and the data from the sensors require data processing to generate a Cartesian map of the array. Acoustic beacons could also be used to provide the needed information but such an approach goes against the covert nature of the hydrophone array.